Non-contact thermometers, such as infrared thermography equipments and radiation thermometers, currently are being used in a wide range of industrial and laboratory temperature control applications. By using non-contact temperature sensors, objects that are difficult to reach due to extreme environmental conditions can be monitored. They can also be used for products that cannot be contaminated by a contact sensor, such as in the glass, chemical, pharmaceutical, and food industries. Non-contact sensors can be used when materials are hot, moving, or inaccessible, or when materials cannot be damaged, scratched, or torn by a contact thermometer, so that these devices are now widely used throughout industry to record and control industrial processes. It is critical for any such non-contact radiation thermometer to be able to accurately convert the output of its detector into a value representing object temperature.
Currently, most common non-contact thermometers, such as infrared thermography equipments, radiation thermometers, or even those IR ear thermometers that are most welcome in SAR period, are heavily rely on blackbody radiation for calibration. However, when the temperature of a standard blackbody radiator is lower than its ambient temperature, by convention effect, gases outside the blackbody radiator will flow into the blackbody radiator for heat exchange and thus cause mist to be formed on the wall of the blackbody radiator. In certain extreme conditions when the blackbody radiator's temperature is lower than 0° C., mist will become frost and ice that is going to cause damage to the blackbody radiator. On the other hand, when the temperature of a standard blackbody radiator is higher than its ambient temperature, also by convention, heat exchanging is going to happen between the gases inside the blackbody radiator and those outside the same. Since the ambient temperature of the blackbody radiator is easily affected by air flow and other environmental factors so that it is varying, such heat exchanging will cause temperature drift to the blackbody radiator. Therefore, it is important to have a stable blackbody radiator that is free from being affected by thermal convention.
Most current non-contact thermometers are calibrated by the use of a cavity-type or plate-type blackbody source. Nevertheless, as those non-contact thermometers are mostly used when materials are hot, moving, or inaccessible, the calibration temperature of those blackbody sources, no matter it is cavity-type or plate-type, are all being configured at a temperature above its ambient temperature. Thus, the aforementioned thermal convention will not cause frost or ice to be formed on the wall of the blackbody source where there can be at most a little moisture. Moreover, in most cases, the temperature of the blackbody source is usually much higher than its ambient temperature that even if there is moisture formed on its wall, the wetness can be evaporated soon enough if the temperature of the blackbody source is high enough. Therefore, by configuring the temperature of the blackbody source at a specific temperature for establishing a heat balance with ambient temperature which should be stabilized, a stable standard blackbody radiator can be achieved.
However, when the fear of SAR in Year 2003 causes the popularization of IR ear thermometers, the accuracy as well as the calibration of those IR ear thermometers is becoming important issues. Consequently, there are more and more researches have focused their studies on middle-/low-temperature blackbody radiators that are used for IR ear thermometer calibration. On such study is a portable blackbody furnace suitable for the temperature calibration of radiation thermometers, disclosed in U.S. Pat. No. 7,148,450. Moreover, some studies come up with a concept of non-plate-type multi-source blackbody radiator as those disclosed in U.S. Pat. Nos. 5,265,958 and 5,756,992. In U.S. Pat. No. 5,265,958, a plurality of block-shaped blackbody sources are used in cooperation with a porous plate for forming a multi-source blackbody radiator suitable for testing thermal imagers. In U.S. Pat. No. 5,756,992, a blackbody simulating apparatus is provided which comprise: a blackbody simulator, for emitting infrared energy in random directions; a collimator, for collecting a portion of the infrared radiation emitted by the blackbody simulator and then redirects the infrared radiation to a dielectric; the dielectric, having a thin metallic coating affixed to its rear surface to form a mirror like surface at its rear surface. Thereby, a primary reflection of the collimated beam occurs at the front surface of the dielectric resulting in a first blackbody image being directed to an infrared imager; while a portion of the collimated beam passes through the dielectric to the metallic coating and is then reflected by the metallic coating to the front surface of the dielectric emerging from the dielectric as a second blackbody image which is also directed to the infrared imager. Additional blackbody images are generated by internal reflections within the dielectric with each blackbody image having a different intensity from the other blackbody images. This results in at least two different radiance levels being supplied to the infrared imager for calibrating the infrared imager. Nevertheless, In U.S. Pat. No. 6,447,160, a blackbody cavity, which is the most common commercial cavity-type blackbody source, is used for calibration of infrared thermometers since it is proven that such cavity-type blackbody source is more stable and has better emissivity than the block-shaped blackbody source.
However, all the aforesaid studies including cavity-type and block-shape blackbody sources did not provide means for preventing thermal convention. Only in TW Pat. No. 467271, a blackbody furnace 11 is provided that can prevent air surrounding the blackbody furnace 11 from even come near to the furnace opening to enter the radiation cavity 12 for generating thermal convention by the use of an extending tube 14 to tightly combine the outer wall of the blackbody furnace 11 with a thermometer 16, as shown in FIG. 1. However, it is obvious that the extending tube 14 should be designed with various sizes and lengths so as to be adapted for calibrating thermometers 16 of various sizes and measuring distances. Therefore, the aforesaid blackbody furnace can be too complicated to be realized. In addition, it is difficult to design an extending tube 14 that can tightly combine the outer wall of the blackbody furnace 11 with the thermometer 16.